This invention generally relates to color picture tubes and, more particularly, to a method and apparatus that provides magnetic shielding in color picture tubes.
A color picture tube includes an electron gun for generating and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate of the tube and is made up of an array of elements of three different color-emitting phosphors. A color selection electrode, also referred as a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam. A shadow mask is a thin sheet of metal, such as steel, that is usually contoured to somewhat parallel the inner surface of the tube faceplate.
The geometries of tension shadow masks typically include the following: (1) a near cylindrical profile with a near circular profile along the x-axis and a linear profile along the y-axis; or (2) a planar or near planar profile with a linear or near linear profile along the x-axis and a linear profile along the y-axis.
The new generation of tensioned shadow masks has a gap on the short sides of the mask between the mask and the mask frame. The gap is the product of the tension mask being mounted to a set of termination bars that are affixed to standoffs that connect the mask to the mask frame. This gap allows a horizontal component of the earth""s magnetic field to effect the propagation of the electron beam. The magnetic field causes a depreciation in performance in the horizontal component areas of the screen of the television set. The amount of magnetic field penetration is known as shielding efficiency. Therefore, improved shielding is needed to eliminate or, at least, mitigate the intrusive magnetic field to produce a commercially viable television tube.
The present invention provides a method and apparatus for providing magnetic shielding in a tension mask of a color picture tube and in particular, a tube with a flat front profile. The apparatus comprises two magnetic shields that are affixed to the short sides of a mask frame assembly. These shields are formed of sheet metal and are rectangular in shape. Each of the shields contains a hole and several slots disposed through the material to aid in aligning and mounting the shields to a mask frame. The shields have a specific length, width, height and thickness, where the thickness of the sheet metal is very small compared to both the length, width and height of the shield.
The method comprises mounting the aforementioned shields to a mask frame. A shield is placed on each of the two short sides of a mask frame. The shields cover a portion of the mask frame that runs from the top of the short sides of the mask frame to the bottom of the short sides of the mask frame. A set of retaining clips are affixed to the mask frame and guide the shield into correct alignment. The retaining clips pass through slots that are disposed in the shield and temporarily secure the shields to the mask frame assembly. A rivet is placed through a hole formed in the shield and a hole formed in the mask frame to permanently affix the shield to the mask frame. The shield is affixed such that it covers the gap produced by the tension mask being mounted to standoffs affixed to the mask frame. Each of the shields covers the gap between the mask frame and the tension mask and extend slightly above or in front of the tension mask assembly. By covering the gap formed between the tension mask and the mask frame, a magnetic field protection system is realized that provides a higher shielding efficiency for the horizontal component of the terrestrial magnetic field. This invention improves shielding efficiency upwards of 40%.